Fat is an important group of our food.You cannot totally ignore fats because:-
It is a good source of energy.1 gm of fat gives 9kcal of energy.
It also provides the essential fatty acids,linoleic and alpha-linolenic acids which are necessary for growth and development.
It act as a carrier for fat soluble vitamins(vitamin A,D,E and K).it also helps in absorption of these vitamins.
It insulates our body from temperature changes and helps to hold our organs in place

What are Fats?
Fat are substances composed of a combination of fatty acids.There are 4 types of fatty acids:-
Saturated Fatty Acids(SAFA)
Polyunaturated Fatty Acids(PUFA)
Monounsaturated Fatty Acids(MUFA)
Trans-Saturated Fatty Acids(TFA)

Saturated Fatty Acids(SAFA)
suppresses LDL-cholesterol receptors,thereby increasing the blood level of LDL-cholesterol!
increases the fat content in the blood vessels,including coronary arteries!
increases the tendency of clot formation within the blood vessels,leading to heart attacks!
Examples of saturated fatty acids are:- myristic acid,lauric acid and palmitic acid.Myristic acid is the most powerful cholesterol raising saturated acid,followed by lauric acid and palmitic acid.
Major sources of myritic acid are:-butter,beef tallow,lard,coconut oil and palm oil.Coconut and palm oil are also rich in lauric acid.

Polyunsaturated Fatty Acids(PUFA)
There are 2 types of Polyunsaturated Fatty Acids(PUFA):-Omega-3 and Omega-6

Omega-3

it lowers the clotting activity of blood
it reduces platelets activity and monocte reactivity
it reduces triglyceride levels
it reduces blood pressure
it reduces homocysteine level in blood.Elevated levels of the amino acic,Homocysteine is a highly potent risk factor for coronary artery disease.(the acceptable range is 5 - 15 umol/L)
it increases the oxidation of LDL cholesterol.This is a minus point for unsaturated fatty acid!

Major sources of Omega-3 polyunsaturated fatty acid are:-Salmon,Atlantic Mackeral and other fatty fish,walnuts,canola(rapeseed)oil,soyabean oil.

Omega-3
inhibits the synthesis of Apo-lipoprotein B in the liver.This is the carrier protien by which cholesterol is absorbed and transported in the body.A reduced Apo-lipoprotein B equates a reduced uptake of cholesterol
it reduces blood cholesterol level
it reduces the fatty deposits in arterial walls
Major sources of Omega-6 polyunsaturated fatty acid are:-soyabean,corn oil and cotton seed oils

Monounsaturated Fatty Acids(MUFA)
reduces the fat deposition in blood vessels
reduces blood clot formation
restores LDL receptor activity and hence lowers cholesterol
Rich sources of Monounsaturated Fatty Acids(MUFA) are:-olive oil,canola(rapeseed)oil.

Trans-saturated Fatty Acids(TFA)
When vegetable oils are hydrogenated to make 'vegetable ghee',polyunsaturated fatty acids are converted to Trans-saturated fatty acids.
increases LDL cholesterol and triglycerides
reduces HDL cholesterol
Major sources of Trans-saturated fatty acid(TFA)are:-a few types of margarines,vanaspati ghee etc.